This invention relates to a cold reduced deep drawing aluminum killed steel. More particularly, the invention relates to a non-aging low manganese aluminum killed steel having a very high average plastic strain ratio produced from a slab having a reduced hot rolling temperature.
It is well known deep drawing steels are characterized as requiring a very high average plastic strain ratio(r.sub.m) of 1.8 or more. Average plastic strain ratio is defined as r.sub.m =(r.sub.0.degree. +r.sub.90.degree. +2r.sub.45.degree.)/4. High r.sub.m values have been achieved by adding various carbide and/or nitride formers, e.g. Ti, Cb, Zr, B, and the like, to steel melt compositions. However, addition of these elements to a melt to produce non-aging deep drawing steel is undesirable because of the added alloy costs. It is also known aluminum killed steel having an equiaxed grain structure and similar high r.sub.m values can be produced by continuous annealing if aluminum nitride is precipitated prior to cold reduction. Batch annealed aluminum killed steel having an elongated grain structure develops r.sub.m values to about 1.8 by precipitating aluminum nitride during the slow heatup prior to the onset of recrystallization during annealing. Unlike batch annealing, aluminum nitride will not precipitate prior to recrystallization during continuous annealing to form high r.sub.m values because the heating rate is too rapid. Precipitation of aluminum nitride prior to cold reduction to produce high r.sub.m values for continuously annealed aluminum killed steel is accomplished by using a high coiling temperature after hot rolling or by reheating a relatively cold slab to a temperature insufficient to re-dissolve aluminum nitride precipitated during cooling of the slab following casting.
The following prior art discloses cold reduced aluminum killed steel produced by continuous annealing. U.S. Pat. No. 4,145,235 discloses a process for producing a low manganese aluminum killed steel having high r.sub.m values by hot coiling a hot rolled sheet after hot rolling at a temperature no less than 735.degree. C. Values for r.sub.m up to 2.09 after continuous annealing are disclosed. U.S. Pat. No. 4,478,649 discloses a process for direct hot rolling a continuously cast aluminum killed steel slab without reheating the slab. The as-cast slab is hot rolled prior to the slab cooling to a temperature below Ar.sub.3 thereby avoiding precipitation of aluminum nitride. Aluminum nitride is precipitated prior to continuous annealing by hot coiling the hot rolled sheet after hot rolling at a temperature of at least 780.degree. C. U.S. Pat. No. 4,698,102 discloses using aluminum killed steel slab reheat temperatures less than 1240.degree. C. so that aluminum nitride precipitated during cooling of the slab following casting is not re-dissolved prior to hot rolling. Coiling temperatures after hot rolling of 620.degree. -710.degree. C. are disclosed to precipitate any remaining solute nitrogen prior to continuous annealing. U.S. Pat. No. 4,116,729 discloses cooling a continuously cast aluminum killed steel slab to within the temperature range of 650.degree. C. to Ar.sub.3 for at least 20 minutes to precipitate aluminum nitride. The slab is then reheated to 950.degree.-1150.degree. C. for hot rolling without re-dissolving the aluminum nitride. Values for r.sub.m up to 1.6 after continuous annealing are disclosed. U.S. Pat. No. 4,627,881 discloses a process for producing high r.sub.m values in continuously annealed aluminum killed steel by controlling the nitrogen to no greater than 0.0025% and the phosphorus to no greater than 0.010% with the sum of phosphorus plus five times the nitrogen no greater than 0.020%. Slabs were reheated and hot rolled within the temperature range of 1050.degree.-1200.degree. C. The hot rolled sheet was coiled at a temperature of less than 650.degree. C. Cold reduced continuously annealed sheet had r.sub.m values up to 2.1. It is also known continuously annealed aluminum killed steel having high r.sub.m values can be produced by increasing the soluble aluminum in the melt. U.S. Pat. No. 3,798,076 discloses an aluminum killed steel having 0.13 to 0.33% soluble aluminum and r.sub.m values up to 1.91 after continuous annealing.
It is also known aluminum killed steel having similar high r.sub.m values can be produced by batch annealing. U.S. Pat. No. 3,959,029 discloses using conventional slab hot rolling practice so as not to precipitate aluminum nitride, i.e. keep nitrogen in solution, prior to batch annealing. Values for r.sub.m up to 2.23 were disclosed for a non-aging aluminum killed steel by decarburizing a cold reduced sheet during annealing to less than 0.01% carbon. U.S. Pat. No. 4,473,411 discloses a batch annealed aluminum killed steel having r.sub.m values up to 1.85. The sheet was produced from a slab using conventional (1260.degree. C. slab drop-out temperature) hot rolling practice having 0.12-0.24% manganese that was hot rolled without precipitating aluminum nitride. The hot rolled sheet was cold reduced and its cold spot temperature carefully controlled during annealing to develop high r.sub.m values.
As previously indicated, addition of carbide and/or nitride forming elements to a melt to produce non-aging deep drawing steel is undesirable because of the alloy costs. Using elevated coiling temperatures to produce non-aging deep drawing aluminum killed steel is also undesirable because of uneven cooling rates and the scale formed on the hot rolled sheet during cooling from the elevated coiling temperature is more difficult to remove. The melt processing techniques, i.e. vacuum degassing, ladle stirring, fluxing, and the like, required to reduce the residual carbon, nitrogen or phosphorus also are expensive. Accordingly, there remains a need for an inexpensive non-aging deep drawing aluminum killed steel. More particularly, there remains a need for a non-aging aluminum killed steel having an r.sub.m value of 1.8 or more that can be produced using conventional processing or using a technique that does not add, and preferably saves, cost over that of conventional processing.